Yol. 51.] FOKMATION OF OOLITE. 199 



the disappearance of the clearly-defined concentric arrangement. 

 On the other hand, it is possible that the concentric arrangement, 

 if it ever existed, has been obliterated by molecular changes. 



The illustration, PI. VII. fig. 6 a, is drawn from a positive 

 photograph, and represents the object as it appears when examined 

 by transmitted light. When the granule is viewed by reflected 

 light the dark portion in fig. 6 a appears white slightly tinged with 

 yellow, and the remaining portion of the object is clear, but not 

 crystalline. These features are well illustrated in PI. VII. fig. 6 b, 

 which is drawn from a negative photograph and represents the 

 object as seen by reflected light. 



It is, then, evident that the granule is made up of calcium carbo- 

 nate existing in two forms : (1) the non-crystalline clear, representing 

 the organic portion of the original granule, and (2) the dark granular, 

 which is really amorphous calcium carbonate (ordinary carbonate 

 of lime), and is either infilling or secreted material, possibly both. 

 In other words, the ' clear ' represents the walls or skeletal structure 

 of the organisms forming the granules ; and the granular has either 

 been secreted by these organisms, as in the case of the calcareous 

 algae, or has filled in spaces left by the decomposition of the living 

 matter. 



PI. VII. fig. 5 shows a type of granule somewhat similar to the last- 

 described. In this granule a portion of the crust exhibits concentric 

 laminae dotted with numerous more or less oval spots. In the re- 

 maining portion the concentric arrangement disappears, and we 

 have a mass of dark l openings or spots, surrounded by a clear form 

 of calcium carbonate, the former suggestive of infilled organic 

 structure. 



PI. VII. fig. 7. — This granule differs from any that I have pre- 

 viously discussed. There is a central nucleus of some sort enclosed 

 by a crust devoid of laminated concentric structure, nor is there any 

 sign of such a structure having ever existed. It cannot, therefore, 

 be said that granules of this typo have originated from chemically- 

 deposited concentric strata. 



PI. VII. fig. 3. — This is a very remarkable type of granule. In 

 the centre is the usual nucleus, the original nature of which is not 

 clear. At first, this is surrounded by an inner crust, which in part 

 shows an obscure mixture of concentric and radiate structure. In 

 some parts the structure is wholly radiate, and among this the out- 

 lines of tubules are visible having the appearance of striae at right 

 angles to the nucleus. 



The inner crust terminates in a very irregular concentric growth. 

 At two points the lamina? diverge considerably, leaving spaces which 

 are filled in with amorphous calcium carbonate. Next, an outer 

 crust appears, also made up of irregular concentric laminae. 



As in previous examples of granules, the calcium carbonate is 



1 White, with a tinge of yellow, by reflected light. 



